1. Field of the Invention
The present invention relates generally to automatic flight control systems for aircraft, and more specifically to an altitude preselect control system for both fixed wing and helicopter aircraft which are not equipped with an air data computer.
2. Description of the Prior Art
The automatic flight control system, or autopilot, is growing in popularity among the owners of smaller, business or general aviation aircraft, and of course, a primary concern of such owners is the cost of the system. To keep cost to a minimum many automatic flight control systems for small aircraft are of the simple primarily pilot relief type, which provide attitude stabilization and minimal maneuver capability. Such a system is described in U.S. Pat. No. 3,848,833, issued to Applicant's assignee. Few systems of this type include an altitude preselect function. Altitude preselect is one of a number of very convenient flight path control functions found in more elaborate commercial and military automatic flight control systems. It permits the pilot to preselect a desired flying altitude, then to put the aircraft into a climb or descent at a rate calculated to maximize fuel economy, for example. The altitude preselect function assumes control of the climb or descent when the desired altitude is approached to smoothly and gently capture the aircraft at the desired altitude and hold it there. In these more elaborate systems as air data computer implements the altitude preselect function. The air data computer is capable of providing a continuous measure of actual altitude and rate of climb or descent for controlling the flight path of the aircraft and for display within the cockpit. But air data computers may be too costly for the small aircraft owner, hence only a few small aircraft have this luxury item.
However, substantially all aircraft using U.S. air space include, by FAA regulations, a pneumatic encoding altimeter for transmitting aircraft altitude through a transponder to ground stations for enroute and terminal traffic control purposes. The pneumatic encoding altimeter is typically of the type described in U.S. Pat. No. 4,133,209 in which the altitude is sensed pneumatically, referenced to standard barometric pressure at sea level (29.92 in. Hg), and transmitted in encoded fashion to a ground station. The altimeter is usually provided with a manually operable calibration control for adjusting a graduated scale plate. By setting the calibration control in accordance with locally reported ambient barometric pressure, the altimeter gives a cockpit indication of actual aircraft altitude. The encoded signal transmitted to the ground station is not affected by the calibration control, and is thus always referenced to 29.92 in. Hg. Changes in altitude of an aircraft carrying the encoding altimeter are accompanied by changes in ambient pressure, and thus cause movement of a pointer over the scale plate and also cause rotation of a shaft coupled to, for example, an optical encoding angular position sensor. The resolution of an encoding altimeter is, by Federal regulation, 100 feet. In other words, the optical encoding angular position sensor produces a signal representing altitude change in 100 foot increments. Because the output of the encoding altimeter is not a smooth and continuous representation of actual instantaneous altitude, such an altimeter has not been used heretofore in an automatic flight control system to provide the altitude preselect function.
Thus it is an object of this invention to provide a low cost altitude preselect apparatus for use with aircraft not equipped with a costly air data computer. Another object is to make the altitude preselect apparatus compatible with existing automatic flight control systems. Yet another object is to utilize existing aircraft instruments and sensors, such as a pneumatic encoding altimeter, to provide instantaneous altitude and vertical speed information needed as control data for the altitude preselect apparatus.